The present invention relates to a controller for fluid pressure operated devices and, more particularly, to a controller having an improved spool-sleeve valve arrangement.
Although the present invention is equally adapted to any controller for fluid pressure operated devices wherein the controller utilizes a valve spool arrangement, it is especially advantageous when used in controllers for power steering systems of the type employed in off-the-road vehicles, and will be described in connection therewith. More specifically, although the invention will be described in connection with rotatable spool-sleeve valve arrangements, it will be appreciated that the invention may also be utilized with spool valves which operate in response to axial movement.
A controller for a power steering system of the type to which the present invention pertains is described in U.S. Pat. No. Re. 25,126, assigned to the assignee of the present invention. Controllers of the type disclosed in the cited reissue patent have become well known in the art and generally comprise a housing having an inlet and an outlet and a pair of control fluid ports, feeding a power steering cylinder. The vehicle steering wheel is directly connected to the controller and when in the neutral (non-rotating) position, fluid may pass from the inlet through the valve to the outlet (open center system), or fluid from the inlet may be blocked from passing through the valve (closed center system).
When the steering wheel is rotated in one direction from the neutral position, the valve is displaced and fluid flows from the inlet through the valve, to the meter, then to one of the control fluid ports to move the power steering cylinder. When the steering wheel is rotated in the opposite direction, the valve rotates in the opposite direction and fluid flows from the inlet port through the valve, then through the fluid meter in the opposite direction, then to the other of the control fluid ports to move the power steering cylinder in the opposite direction.
Conventionally, controllers of the type described have utilized rotary spool-sleeve valves to direct the flow of fluid from the inlet port in accordance with the rotational position of the steering wheel. In general, rotary spool-sleeve valves comprise a primary valve member (spool) connected directly to the steering wheel and a follow-up valve member (sleeve) surrounding the spool. Axially adjacent the spool and sleeve is a fluid meter, generally a gerotor having an externally toothed member orbiting within an internally toothed member. The externally toothed member is splined to a drive shaft, at the opposite end of which the drive shaft is coupled to the sleeve, such as by a pin passing therethrough. When the spool is rotated, fluid is permitted to flow to the meter, causing the externally toothed member to orbit and rotate, thus imparting rotary follow-up movement to the sleeve by means of the drive shaft. Generally, the sleeve has a plurality of orifices extending radially therethrough and the spool has a plurality of axially extending grooves on its outer surface to provide communication between certain of the orifices in the sleeve.
One of the problems associated with conventional power steering systems and the controllers used therein is steering wheel "precession", i.e., the position of the steering wheel corresponding to the neutral position of the controller "precesses" or moves slowly in one direction or the other during operation of the system. It is believed that this is caused primarily by imbalance in the fluid flow paths, i.e., the fluid is subjected to a longer path and/or more flow restriction for one direction of steering than for the other. Among the other problems associated with systems and controllers of the type described is internal leakage, primarily between "metered" fluid and "return" fluid. As used herein, the term "metered" fluid refers to fluid which has been measured by the fluid meter and is then fed to the power steering cylinder. The term "return" fluid refers simply to fluid displaced by the movement of the power steering cylinder which returns to the valve and passes to the outlet port (or tank port). While the above definitions are strictly true only for a controller in which the flow order is: inlet port-- fluid Meter-- cylinder-- outlet port, it will be apparent that the invention is equally adapted for controllers having other flow orders, including but not limited to: inlet pot--cylinder-- fluid meter-- outlet port. In connection with the definitions of "metered" and "return" fluids for any particular controller, it should be noted that the remainder of the fluid passing through the spool-sleeve valve is at approximately the same pressure as the "metered" fluid, but has not been metered and hence, may be referred to as "high pressure, non-metered" fluid.
In many of the conventional spool-sleeve valve arrangements, the orifices in the sleeve communicating to and from the control fluid ports and the grooves in the spool communicating therewith are arranged in such a manner that grooves containing metered fluid and grooves containing return fluid are in an alternating, interdigitated relationship with each other, thus greatly increasing the length of the interface between metered and return fluid and the opportunity for internal leakage therebetween (see, for example, U.S. Pat. No. 3,819,307).